Oil and gas wells are often stimulated in order to enhance production. Stimulation techniques are employed to make marginal wells economically feasible for continuing production. Most stripper wells would not be financially viable absent the advances in stimulation techniques available in the art today. Although there are numerous stimulation methods that have gained credence in the art, the most widely used methods involve the use of explosives, acidizing, and fracturing.
One explosive approach is nitro-shooting, which involves the use of explosive detonations at select subterranean locations to shatter and fracture a geological formation. The fractured formation is thusly provided increased permeability, resulting in increased production capability. The use of subterranean explosives is almost as old as the oil industry itself. More recently, the use of explosives has given way to the use of guns that fire bullets into the formation. This enhances the ability to direct the fracturing relative to a desired fault line, the results being more predictable than the relatively random effects of underground explosives.
Acidizing has 19th century origins and was fully recognized as a viable well stimulation method by the 1930s. Acidizing involves subterranean injection of acid into an acid-soluble formation where the dissolving action enlarges openings in the formation to increase permeability. Many techniques are employed to localize and direct the dissolving reaction, such as packing or cementing portions of the casing to isolate the treatment region. Acidizing has more recently been widely replaced with hydraulic fracturing.
First introduced in the late 1940s, hydraulic fracturing sparked immediate and widespread acceptance as a viable method of increasing the permeability of a well. Unlike acidizing, fracturing is effective in any type of formation and can be combined with acidizing for even better results in certain formations. This method consists generally of injecting a fracturing fluid and propping agent mixture into the formation and applying pressure, typically a hydraulic pressure, to reopen existing fractures and create new ones. The propping agent commonly is an appropriately sized silica sand.
After the hydraulic pressure has forced open the fractures in the formation, a portion of the propping agent remains wedged in the fractures to hold the fractures open. The oil and gas subsequently produced thereby flows through the embedded propping agents that remain wedged in the fracture.
All of these stimulation methods have in common the need to clean the stimulated area, drill string, and casing before resuming oil and gas production. Debris from explosives--residue from acidizing and propping agent particles from fracturing--similarly must be flushed from the well prior to resuming production. The well formation pressure is used to expel the debris and residue; that is, prior to returning to service, the well is first operated to produce a flush stream to flush the debris from the well. The mixture of oil and gas products with debris, otherwise referred to as a flush effluent, must be disposed in an appropriate manner. In the past, the flush effluent was commonly discharged into open earthen pits where the gases and liquids evaporated and the solids were then buried or hauled away for disposal.
Concerns about pollution leaching into the soil and jeopardizing potable water tables have stimulated state and federal legislation to effectively eliminate the use of earthen pits for storing the flush effluent. Today, in most cases, the flush effluent must be handled as a hazardous waste material. The current common practice is to collect the flush effluent as a whole, that is, as a mixture of subterranean fluids and debris. The flush effluent is typically delivered into a storage container, such as a frac tank. There are, however, generally recognized difficulties associated with collecting and hauling off the flush effluent as a whole.
One problem is associated with the erosive nature of the flush effluent. The formation pressure that delivers the flush effluent is typically high, and the propping agent sufficiently abrasive, so that the flush effluent stream can quickly cut away the steel floor of a frac tank. Extensive monitoring and frequent repair of the tanks is necessary to prevent catastrophic leaks. Should a tank fail while in transit, waste spills can result in public areas, thereby creating unacceptable environmental hazards.
Another problem is associated with the relative expense involved with collecting the effluent as a mixture rather than as separated constituent parts. By separating the effluent into constituent parts before disposal, significant improvements in operating efficiency are realized. It would be desirable to perform an on-site separation, but commercial separators available in the art, those typically used in other operations such as the recovery of drilling fluid, are inherently incapable of separating the effluent at hand given the characteristic throughput requirements.
For example, U.S. Pat. No. 5,718,298, issued to Rusnak, teaches a separator for separating the constituent parts of gases and solids in a drilling fluid where air is the primary constituent. Since the separator of Rusnak '298 separates two constituents, it is commonly referred to as a two-stage separator. Three-stage separators, capable of separating solids, liquids and gases, are similarly employed to dispose of drilling fluids, such as those taught by U.S. Pat. No. 5,415,776 issued to Homan and U.S. Pat. No. 4,247,312 issued to Thakur et al.
One skilled in the art will recognize that these and other commercial separators available in the prior art are not suited for the characteristics of an effluent flow like that of concern in the present invention. The velocity and abrasiveness of the effluent resulting from a fracturing procedure is far too aggressive to be operatively controlled by the structural components of these devices. Particularly, the involute of Homan '776 and the deflector plates of Rusnak '298 would be quickly eroded and thus rendered ineffective under the conditions of recovering fracturing materials from a well. Thakur '312 teaches a separator having a relatively low velocity inlet stream wherein the solids settle out directly beneath the inlet. This is an unworkable solution because the relatively high velocity of the effluent stream of present concern is too aggressive to succumb to the settling action of solids and weiring action of liquids provided by the separator of Thakur '312.
Although U.S. Pat. No. 5,513,704 issued to Sander makes provision for the relatively aggressive characteristics of a flush effluent of present concern, there is lacking the capability for three-stage separation of the flush effluent. The gas constituent is separated from the flush effluent, but the solids and liquids are removed from the collection vessel as a mixture and must therefore be separated remotely from the collection vessel. As such, there is lacking in the prior art any solution that provides a self-contained apparatus for the separating of all constituents, that is, solid, liquid, and gas constituents, before removal thereof from the receiving vessel.
A more serious problem lies in the flammable propensity associated with methods of collecting the flush effluent. The flush effluent typically contains flammable hydrocarbons, both liquid and gaseous, and also contains subterranean fragments and fracturing materials that are delivered at a high velocity into the storage tank. The impacting fragments against the steel tank often create sparks, which in the presence of combustion air and the hydrocarbons can result in a fire or explosion. Today it is a common precaution to keep a ready supply of drilling mud on hand for the sole purpose of extinguishing a fire resulting from these conditions. This reactive approach to a known dangerous condition has tragically harmed numerous oil well personnel. Even assuming that injuries could be prevented, the property losses stemming from fire damage to wells and equipment is significant. The health, safety, and equipment costs associated with meeting the environmental concerns at hand are widely recognized as significant. These costs justify the use of a separator that eliminates the possibility of fire or explosion in recovering the effluent.
The Sander '704 separator fails to contemplate the flammable nature of recovering the effluent of present concern. Sander '704 teaches the use of an impact deformable member, such as a green hardwood log, to divert the effluent flow in a receiver vessel. The propants and the debris in the inrushing effluent embed in the deformable member, providing an erosion resistant diverter surface. Although this may solve the erosion problem associated with recovering the effluent at hand, the propant and fragments that subsequently strike the embedded propant and fragments will create sparks, and the sparks are in the presence of combustion air and fuel.
Hence Sander '704 fails to resolve the inherent safety concerns associated with recovery of the effluent at hand.
There is, therefore, a need in the industry for an improved separating apparatus to collect the flush effluent from a well-site stimulation, such as the fracturing material used in hydraulically fracturing a formation to increase permeability. The apparatus would advantageously capture the effluent as delivered at the formation pressure and process the effluent to separate the constituent parts and safely dispose of the constituents in a self-contained manner.